This proposal is a direct continuation of our previous grant directed at the identification, mapping and characterization of genetic factors affecting lipoprotein metabolism and arterial fatty streak lesions using a mouse animal model. Each of the previously defined specific aims was addressed in detail, and these results have raised the following questions which form the basis of this proposal. (1) Studies of naturally occurring genetic variations of the apolipoprotein A-II (apoAII) gene in mice indicated that apoAII expression determines high density lipoprotein (HDL) size, composition and levels, and they further suggested that increased apoAII expression resulted in increased aortic fatty streak lesions in mice-maintained on an atherogenic diet. These conclusions were tested by constructing transgenic mice overexpressing apoAII, and the results were entirely consistent with the genetic studies. An important surprise was that such transgenic mice developed aortic fatty streaks even when maintained on a low fat, chow diets. The mechanism by which apoAII expression controls lipoprotein metabolism and arterial lesion development will now be further explored using biochemical approaches, gene modification, and genetic analysis. (2) Techniques for quantitative trait locus (QTL) mapping of complex genetic traits (traits determined by multiple genetic factors) were established during the previous giant period. QT,I- mapping is being applied to the analysis of lipoprotein metabolism and atherogenesis in several large genetic crosses between strains of mice differing in lipoprotein metabolism. Thus far, several loci controlling lipoprotein metabolism have been identified. Some of these loci correspond to the positions of known candidate genes (for example, LDL levels segregate with a chromosomal region containing the gene for cholesterol 7a-hydroxylase), whereas others are novel. These results will be confirmed and extended, and molecular analyses of these loci will be pursued using combined biochemical-genetic approaches, including positional cloning. Hypotheses will be tested in vivo using transgenic and gene targeting technologies. (3) We have also identified a novel genetic abnormality in mice characterized by dramatically elevated plasma triglyceride levels (about 34O mg/dl) and somewhat elevated cholesterol levels. The genes responsible for the trait (most likely involving an interaction of two unlinked genes) will be mapped and characterized. The results will provide new candidate genes for human hypertriglyceridemia and familial combined hyperlipidemia.